Draw the figure.
draw a line from X angle perpendicular to side x.
Note then that zsinY=ysinZ or
z=ysinZ/sinY
now draw a line from angle Y to side y perpendicular to y.
Note that area=1/2 bh=1/2 y*zsinX
then area= 1/2 ysinX*ysinZ/sinY
QED
prove that the area of any triangle XYZ is y^2*sin(x)*sin(z)/2*sin(y)
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